[incognito]

I think, if you say it's ok when you point down, it works better, I assume it is because of the forces being increased with gravity, enough to spray well, and you queried your air pressure, have you tried increasing the pressure a tad to compensate, achieving the force necessary for smooth level spraying similar to what you get pointing downwards, i.e. with the gravity added?

I would try that first on a bit of rough paper or something, also, as KDsilverbrush suggests checking for the tip dry possibility.
As an added thought, it might be you have a tiny obstruction in the ab, bit of microscopic dried paint, which again the extra gravity is able to overcome, but level it doesn't, check the needle, and nozzle for cleanliness.

Hope that works out for you,
JonO